1. Field of the Invention
The present invention relates to a high-vacuum-maintaining structure of a superconducting cable, and more particularly to a high-vacuum-maintaining structure of a superconducting cable which enables simple and uniform removal of residual gas inside a superconducting electric power transmission cable having a considerably long length, thereby maintaining the interior of the superconducting cable in a high-vacuum state for a long time, as well as improving thermal insulation performance of the superconducting cable, and achieving reductions in cooling costs and maintenance costs required for vacuum pumping.
2. Description of the Related Art
In general, the term “superconductivity” indicates a phase having a substantially null resistivity in a cryogenic state, such as, for example, below a given temperature of −196 degrees centigrade, and the term “superconducting cable” indicates an electric power transmission cable made of certain conductors exhibiting superconductivity.
The superconducting cable comprises a cable core having one or more tapes of superconductors and insulators wound on a former, defining a liquid nitrogen flow channel and a cryostat surrounding the cable core to define the liquid nitrogen flow channel and serving as a vacuum thermal insulation layer.
Referring to FIG. 1, illustrating a conventional cryostat designated as reference numeral 1, it is thermally insulated by maintaining a high-vacuum level between an inner metal tube 10 and an outer metal tube 15 thereof, taking the form of corrugated tubes. Between the inner and outer metal tubes 10 and 15 there are taped multi-layered insulators (not shown) in order to obstruct heat from being exchanged between a normal ambient temperature and a cryogenic temperature of liquid nitrogen.
Furthermore, in order to prevent heat-contact between the inner and outer metal tubes 10 and 15 and the multi-layered insulators interposed therebetween, and to concentrically maintain the inner and outer metal tubes 10 and 15, the cryostat 1 comprises spacers 20 taped between the inner and outer metal tubes 10 and 15. Conventionally, the spacers 20 are in the form of spirally-wound Teflon™ or polyester wires placed between the inner and outer metal tubes 10 and 15.
One of the problems of the conventional cryostat 1 is that it is difficult to maintain a high vacuum level between the inner and outer metal tubes, 10 and 15 due to residual gas (G) generated from the inner and outer metal tubes 10 and 15 as well as the insulators.
Since general low-temperature containers for use in other technical fields have a relatively short length, the residual gas (G) generated therein can be easily removed, as the low-temperature containers are washed by an ultrasonic washing technique or are heated in a furnace when being manufactured. However, in case of the cryostat 1, constituting the superconducting electric power transmission cable, due to a considerably long length thereof equal to hundreds of meters, there is difficulty in removing the residual gas (G) therefrom using the ultrasonic washing technique or heating in a furnace.
If the residual gas (G) generated in the cryostat 1 is not removed, the cryostat 1 suffers from deterioration in vacuum level and thermal insulation performance thereof, resulting in a problematic increase in cooling costs and maintenance costs required for periodic vacuum pumping operations.
Additionally, there is a concern that the residual gas (G) may affect the critical temperature of the superconductors, thereby deteriorating electrical signal transmission properties of a system.